Antenna Arrangement

ABSTRACT

An antenna for use in a direction finder system, said antenna including a microstrip patch layer with a stripline transmissionline layer below. The microstrip layer and the stripline layer share a common groundplane. In this groundplane is formed a number of apertures. Each aperture couples a patch to a radiation point in the stripline feed layer. The patches and transmissionlines are designed from a lossy material. The substrates in the microstrip and stripline layers may be designed from materials with lossy dielectric properties.

TECHNICAL BACKGROUND

The present invention discusses a novel structure for an antenna unit in a direction-finder system for detecting and localising radio frequency emitters. The present solution makes use of an antenna structure which is extremely stable and which gives a very accurate direction to any number of emitters.

Radio frequency emitters (radars, satellite uplink stations, cell-phone base stations, relay links) can be detected, analysed, and geo-referenced from a remote observation platform. This is achieved using a sensor with an antenna system for detecting the radiation, connected to a receiver and processing system. These systems can be deployed from satellites, aircraft, UAVs, ships vehicles or mounted in masts.

Typical solutions employ radio receiver systems operating in the frequency bands 1 through 12 GHz. These systems employ multiple receiving antennas and multiple receivers to derive a course direction to the emitters.

In general, existing systems either gives high accuracy or wide angular coverage, and have the following limitations:

Angular accuracy is limited (typically 1 deg) Limited ability to handle multiple emitters Large physical size

Poor capability to operate in adverse conditions (e.g. Sun radiation, vibration etc.)

Use of multiple separately supported antenna structures Expensive manufacture SUMMARY OF THE INVENTION

The overall object of the present invention is to provide an antenna system for use in direction-finding systems, which it is optimised for direction finding accuracy and ability to separate individual emitters.

Another object of the inventive antenna system is to provide both high accuracy and wide angular coverage.

Another object is to provide an antenna system with high fidelity wave front pickup and optimized phase linearity.

Still another object is to provide an antenna system with optimal mechanical, thermal and electrical stability.

The objects above are achieved in an antenna system as claimed in the appended patent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in reference to the appended drawings, in which:

FIG. 1 is a system overview illustrating the inventive antenna arrangement in use in a direction-finder system,

FIG. 2 is a perspective drawing showing a first embodiment of the inventive antenna arrangement,

FIG. 3 a shows a patch antenna layer of the antenna illustrated in FIG. 2,

FIG. 3 b shows the feed-line layer of the antenna illustrated in FIG. 2,

FIG. 4 shows the layered design of the antenna of FIG. 2 in cross section,

FIG. 5 shows an alternative design of the antenna, in cross section,

FIG. 6 shows how an antenna according to the invention will deform when subjected to temperature changes,

FIG. 7 shows actual measurements of the distortions of an antenna subjected to a temperature swing of 67 degrees,

FIG. 8 illustrates an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Initially, we will give an overview of the direction-finder system as a background for the present invention. As shown in FIG. 1, the direction-finder system includes an antenna unit 1 at left receiving RF signals from a number of emitter sources. The signals are delivered to an RF unit 2, center, where they are amplified, transposed down to baseband and demodulated. The demodulated signals are delivered to a processing unit 3 for processing and analysis.

The antenna is organised as 4 sub-panels mounted in a 2×2 configuration with a spacing of approximately one quarter of a wavelength.

Typical dimensions of the antenna sub-panels is 1-5 wavelengths (15 cm-75 cms at 2 GHz)

The antenna has an electrical layout optimized to achieve an accurate response in terms of phase errors and mutual coupling effects. To minimize the impact of thermal and mechanical distortions, e.g. warping due to sun heating, the antenna has been designed as a layered composite structure with a stiff support layer. This restricts the electrical solutions that are available for this design. Of many possible designs that have been considered, two constructions have been found advantageous in this connection: Patch antenna array with stripline feed, and slot antennas fed from slotted waveguides.

The first option has been found easiest to implement in a layered design, and is the preferred embodiment of the invention. We will describe this solution, illustrated in FIG. 2 from the outside in, i.e. from the outside receiving the radio signal, to the signal is leaving the antenna element in a feed-line below. The outer layer consists of a number of microstrip patches 11 on a dielectric substrate 12 and a conductive ground plane 13. The layout of patches is illustrated in FIG. 3 a.

In order to optimize the patches for high fidelity wave front pickup and wide bandwidth, the patches are designed to be as wide as possible, room permitting, and being of a lossy conductive material.

The patches may be widened by thickening the substrate while lowering the substrate relative permittivity. In this way the value of the impedance of the patches against the groundplane is preserved.

The patches are also designed to transform the impedance of free space (370 ohm) into the impedance of the feed-line transferring energy from the patches to the RF unit 2.

The patches, and the whole feed structure are designed to introduce ohmic losses into the signal path. The preferred approach for introducing losses, is to cover the copper conductors in patches, groundplanes and transmission lines with a lossy material. As an alternative, or in addition to the former approach, the substrate may have dielectric losses. Still another alternative is to produce the patches, groundplanes and transmission lines from a semiconducting material instead of copper.

Typical values for conductivity of the lossy material are about 0.2 Siemens/m or less. As for the substrates, the loss angle should typically be above 0.1 radian.

The introduced attenuation tends to swamp internal and external reflections in the structure, and is an important feature for obtaining the desired response. The attenuation also masks any residual impedance mismatch against free space, and effectively couples the patches to this impedance. This gives a hereto superior electrical response in terms of phase and amplitude accuracy. The penalty of this is that the antenna has an insertion loss of 2-5 dBb, which is of no consequence for passive reception of signals. In fact, a gain in dynamic range of about 40 dB is obtained instead.

Below the patch layer, is a stripline feed layer composed of a number of narrow flat conductors 14 lying between upper 13 and lower 15 ground planes. The upper ground plane 13 is also acting as the ground plane for the microstrip patches 11.

The layout of stripline conductors 14 is illustrated in FIG. 3 b. The conductors 14 are branching out from a common connection point 17. The striplines are terminated in a connector or an integrated phase comparison network. In the other end, each stripline is terminated in a “radiation point”, where signals are entering the conductor. As can be seen from the figure, the layout of the conductors is strictly symmetrical, resulting in identical travel paths for the signals from each radiation point. This design is chosen in order to minimize phase errors. The striplines are unbalanced in nature, but the conductors are arranged in a symmetric pattern branching out from two connectors forming the connection point 17. The connectors are connected to the balanced port of a balun (not shown), which has the unbalanced port connected to a coaxial cable. Thus the unbalanced striplines are organized in a balanced pattern which is connected to a balun which transforms the balanced signal into an unbalanced signal for the unbalanced cable.

In the stripline pattern, there is one “radiation point” for each patch 11, located below the patches 11, in a one-to-one relationship. In the upper groundplane 13 there are a corresponding number of apertures 18 transferring RF energy from the patches 11 to the associated radiation points. This design, with aperture coupled patches, works to isolate the patches from each other by restricting spreading of RF energy between adjacent patches.

As in the patch layer, the conductors 14 and/or the dielectric material between the groundplanes 13, 15 and the groundplanes themselves should be lossy.

FIG. 4 shows the antenna in cross section. The antenna includes a lower layer 31 of a structural skin material, e.g. a fibreglass/cyanate ester product marketed as Neltec N8000. The layer may be 1-2 mm thick. Above this layer is a panel structural layer 32 of a honeycomb material, e.g. a product known as HRH 327, which in this example is about 25 mm thick. The layer 32 provides structural strength to the antenna, and is responsible for its stable mechanical properties. Above this is a second structural skin layer 34. This may also be made from Neltec N8000 in about 1 mm thickness. The structural skin layers 32, 34 are included to close the cells of the honeycombs. Then there is a layer of double sided circuit board with the conductor pattern (14 in FIG. 2) printed on its lower side, and the upper conductive layer acting as upper ground plane of the stripline layer with the apertures (15 in FIG. 2). This layer may be produced in 1 mm thickness and from the commercial stock board Roger's 5880. Above this is an alumina enhanced thermal barrier layer 35 of 1-2 mm thickness. This layer forms the dielectric layer for the microstrip patches, and is given a low dielectric constant to increase its thickness. A thin dielectric sheet 36 of Kapton (0,025 mm) forms a support for the microstrip patch radiators, here made from copper.

In order to bring stable thermal properties to the panel, the layers in FIG. 4 are designed with a balanced thermal expansion around a symmetry plane along the middle of the structural support layer. An alternative approach for obtaining the same effect would be to duplicate the whole layer structure. This means that a mirror image of the layers shown in FIG. 4 should be glued onto the lower layer 31. This ensures an identical thermal expansion/contraction in the two structures, which balances each other and prevents the panel from bending.

An alternative embodiment of the invention is shown in FIG. 5. Compared with the embodiment shown in FIG. 4, the structural skin layers 31, 33 has been replaced with layers 41, 43 of graphite cyanate ester, e.g. the commercial material known as T300/CE3. In addition, the structural support layer 42 is made of ribs of “SNAPSat”, a material marketed by COI.

In order to diminish thermal heating from the sun, the antenna may be covered with a PVC film with anti-glare properties. When used in space, the antenna may be mounted behind a window of a similar material.

The antenna will when exposed to temperature stress be exposed to the following sources of deformation:

-   -   Antenna stretching (in-plane homogenous dimension change)     -   Antenna bending (out-of-plane structural change)     -   Antenna surface irregularities (out-of-plane)

In addition, depending on the mechanical interface to the surrounding structure, the antenna may experience

-   -   Out-of-plane multi-axis bending (Warping)     -   In-plane irregularities (multiple local in-plane dimension         changes)

The various forms of distortions are illustrated in FIG. 6.

FIG. 7 shows the resulting out-of-plane distortion measured on a sample antenna according to the present invention. The measurements relates to a temperature swing of 67 degrees (from 295 K to 228 K). Peak distortion at the edges is some 30 um, but the symmetrical design and the window function of the antenna illumination brings the actual displacements of the active aperture to well below 10 um.

FIG. 8 shows an alternative embodiment of the invention employing an outer layer 51 with slot radiators 52. Below this layer is a feed structure of waveguide elements 53. Signals are admitted from the slot antennas 52 in the outer layer 51 to the waveguides 53 by a number of series fed waveguide slots 54. This is an alternative antenna with good electrical properties. However, the structure is thick and not as mechanically and thermally stable as the embodiment shown in the previous figures.

As mentioned above, the introduction of lossy materials/lossy dielectrics is important for achieving the is desired phase linear properties of the antenna. In the examples mentioned so far, the loss has been more or less continuously distributed in the antenna. However, it is also possible to use lumped resistive elements instead, forming attenuators in the signal path.

The panels should also be designed to be symmetrical about a central point in the 2×2 configuration. 

1-10. (canceled)
 11. An antenna arrangement in a direction-finder system, said arrangement including a first layer containing a multitude of radiation elements, a second layer containing at least one feed-line, said at least one feed-line being coupled to said radiation elements and being terminated in a connection point, said at least one feed-line being adapted to transfer energy from said radiation elements to said connection point, characterized in that said radiation elements and/or said at least one feed-line includes attenuation elements.
 12. An antenna arrangement as claimed in claim 11, characterized in that said attenuation elements includes lumped resistive elements.
 13. An antenna arrangement as claimed in claim 11, characterized in that said radiation elements and/or said at least one feed-line are designed of lossy materials.
 14. An antenna arrangement as claimed in claim 13, characterized in that said radiation elements and/or said at least one feed-line are designed in copper with a lossy covering layer.
 15. An antenna arrangement as claimed in claim 13, characterized in that said radiation elements and/or said at least one feed-line are designed in semiconducting materials.
 16. An antenna arrangement as claimed in claim 11, characterized in that said radiation elements and/or said at least one feed-line includes dielectric substrates designed in materials with dielectric losses.
 17. An antenna arrangement as claimed in claim 11, characterized in that said first layer consisting of a layer of microstrip patches including a number of individual patches on a dielectric substrate on a first groundplane, said second layer consisting of a stripline transmission line layer located below said microstrip patch layer, said stripline transmission line layer including a number of conductors sandwiched between said first groundplane and a second groundplane, with layers of dielectric substrates interleaved between said conductors and said groundplanes, said conductors including a number of radiation points and lines transferring radio frequency signals from the radiation points to said connection point, said first groundplane including a number of apertures, each aperture being located between an associated patch and an associated radiation point.
 18. An antenna arrangement as claimed in claim 17, characterized in that said conductors are arranged in a branching pattern with the lines from each radiation point to the connection point being of identical length, and preserving an equal phase relationship between signals arriving from the individual radiation points.
 19. An antenna arrangement as claimed in claim 18, characterized in that the lines are organised in two balanced branches, each branch being terminated in one of two connectors in said connection point, the connection point being connected to a balanced port of a balun, the unbalanced port being connected to an unbalanced transmission line conducting the signals to a receiver.
 20. An antenna arrangement as claimed in claim 11, characterized in that said first layer consisting of a slot radiator layer including a number of slot radiators, said second layer consisting of a number of waveguide elements located below said slot radiator layer, said waveguide elements including a number of series fed waveguide slots transferring radio frequency signals from the slot radiators to said connection points. 